1. Field of the Invention
The present invention relates to video imagery and more particularly to apparatus and techniques for providing enhancement of color video images.
2. Description of the Prior Art
Various types of video enhancement apparatus and techniques have been proposed. Automatic Gain Control (AGC) techniques have been employed, inter alia, in video signals for reducing the dynamic range of the video signal by subtraction of its DC level. Such a technique is described in U.S. Pat. No. 4,719,350, wherein linear AGC is applied to the video image on a pixel by pixel basis. This technique is applicable only to a solid area which is delimited by a line parallel to the scan direction.
Generally speaking, automatic gain control is employed in video processing only on an interframe basis and not on an intraframe basis.
Additionally, as described in Stockham, "Image Processing in the Context of a Visual Model", Proceedings of the IEEE, 60(7), July 1972, pages 828-842, and Xie, Stockham, "Towards the Unification of Three Visual Laws and Two Visual Models in Brightness Perception", IEEE Trans Systems, Man Cybernetics, 19(2), March/April 1989, pages 379-382 (also, see, Gonzalez, Digital Image Processing, Second Edition, Addison-Wesley, pp. 185-186), homomorphic filters as shown in FIG. 3 herein, are used for enhancement of black and white images. The logarithm of the input is calculated by log module 501. The output of log module 501 is received by filter 502, which has a frequency dependent gain which may be either linear or non-linear and which tends to decrease the low frequencies and amplify the high frequencies. The output of filter 502 is received by exponent (or inverse log) module 503. This approach separates the illumination and reflectance components. The illumination component of an image generally has slow spatial variations while the reflectance components tend to vary abruptly, particularly at the junctions of very dissimilar objects. Therefore, homomorphic filters emphasize the detail at the expense of low frequencies. Enhancement of the high spatial frequency of the image (details) by a factor of two and attenuation of the DC component down to half of the original value are typical results of such a method.